Electric motors comprise a rotor and a stator having a plurality of wound field coils. Nowadays, brushless DC motors are advantageously implemented in applications of middle and high speed ranges. Brushless DC motors may be of a type of variable reluctance, or a permanent magnet, or a combination thereof. Variable reluctance brushless motors have an iron core rotor that follows or chases sequentially shifting magnetic fields of the stator coils to facilitate rotational motion of the rotor. Permanent magnet brushless motors have the sequentially energized field coils that attract or repel a permanent magnet rotor to facilitate rotational motion of the rotor.
Multiple phase motors typically comprise a permanent magnet rotor and three electrical windings. The three electrical windings are related to the three phases of the motor. Three phase currents flow through the motor windings, typically at a 120 electrical degree phase relationship with respect to one another. The phase currents create a rotating electro-magnetic field which causes angular motion of the permanent magnet rotor.
The control of a brushless DC motor is arranged to apply the power to the motor phases by energizing and de-energizing the individual phase windings. This method is well known as commutation. In order to drive the rotor in a direction from startup and to maintain a desired rotational speed and torque at steady state, a commutation sequence or scheme is applied according to the current rotor position. By this measure, the proper phase windings are energized at appropriate times causing mutual attracting and/or repelling of the phase windings and the rotor magnetic poles, causing the desired angular motion of the permanent magnet rotor.
For a proper rotational movement of the rotor, the control of a brushless DC motor requires information of the position of the rotor with respect to the stator windings. By knowing this position, the control may energize the stator windings in the appropriate sequence to apply a revolving magnetic field in the motor to generate the required rotational torque on the rotor. For detecting the rotor position, it is well known to use measurement data gathered by an external electrically powered speed transducer and/of a speed sensor to sense the position of the rotor with respect of the stator windings.
The use of such transducers and/or sensors is costly because it relates to an increased number of parts with respect to the motor apparatus per se. The increased number of parts contributes to a lower reliability of the motor control since there is a chance to fail under some conditions. Moreover, these additional parts require a valuable space within the motor housing. Since decades it has been the purpose to minimize space consumed by the motor structure and to improve the cost efficiency. In order to make the external electrically powered speed transducer and/or the speed sensor superfluous, several attempts have been made to obtain commutation position feedback without the use of transducers and/or sensors.
For measuring the position and/or speed of the rotor with respect of the stator, a digital, sensorless back-EMF (Electric Motive Force) control technique is common use. This technique is based on back-EMF voltage measurement, is reliable and requires a relatively simple implementation comprising a relatively small additional part being a circuit. In multiple phase motors having a permanent magnet rotor and three electrical windings a, b, and c, the corresponding back emf components are assumed to be a function of rotor position, motor winding current, and rotor speed. This technique is implemented in several applications for operating a brushless direct current motor control at middle and high speed ranges. The back-EMF technique may be used from 5-10% of nominal speed of the brushless DC motor.
In standstill condition and at low speed operation however, the back-EMF voltage amplitude is very low or zero, and thus the position of the rotor is immeasurable. U.S. Pat. No. 6,555,977 describes an apparatus and method for determining the position of the rotor at low speed and even zero speed by deriving a value of the mutual inductance disposed between a first and a second phase by voltage measurement of a floating phase, or unconnected phase, or, as described herein, a third phase, since the voltage of the floating phase is a function of the rotor position due to the mutual inductance components. The patent proposes, however, a voltage measurement of the third phase by using zero crossing and/or detecting a polarity change in the measured mutual inductance, followed by filtering out low frequency components caused by the back-EMF component, followed by rectifying the signal, and followed by filtering out high frequency components caused by a high frequency excitation component.